Numerous peptide and polypeptide ligands have been described to function by interacting with a receptor on a cell surface, and thereby stimulating, inhibiting, or otherwise modulating a biological response, usually involving signal transduction pathways inside the cell that bears the said receptor. Examples of such ligands include peptide and polypeptide hormones, cytokines, chemokines, growth factors, apoptosis-inducing factors and the like. Natural ligands can be either soluble or can be attached to the surface of another cell.
Due to the biological activity of such ligands, some have potential use as therapeutics. Several peptide or polypeptide ligands have been approved by regulatory agencies as therapeutic products, including, for example, human growth hormone, insulin, interferon (IFN)-α2b, IFNα2a, IFNβ, erythropoietin, G-CSF and GM-CSF. Many of these and other ligands have demonstrated potential in therapeutic applications, but have also exhibited toxicity when administered to human patients. One reason for toxicity is that most of these ligands trigger receptors on a variety of cells, including cells other than those that mediate the therapeutic effect. For example, when IFNα2b is used to treat multiple myeloma its utility resides, at least in part, in its binding to type I interferon receptors on the myeloma cells, which in turn triggers reduced proliferation and hence limits disease progression. Unfortunately, however, this IFN also binds to numerous other, normal cells within the body, triggering a variety of other cellular responses, some of which are harmful (e.g. flu-like symptoms, neutropenia, depression). A consequence of such “off target” activity of ligands is that many ligands are not suitable as drug candidates. In this context, “off target activity” refers to activity on the ligand's natural receptor, but on the surface of cells other than those that mediate therapeutically beneficial effects.
Even though some ligands, such as IFNα2b, are approved for the treatment of medical conditions, they are poorly tolerated due to their “off target” biological activity. The off-target activity and associated poor tolerability also mean that some of these peptide ligand-based drugs cannot be administered at sufficiently high dosages to produce optimal therapeutic effects on the target cells which mediate the therapeutic effect.
Similarly, it has been known since the mid-1980's that interferons, in particular IFNα, are able to increase apoptosis and decrease proliferation of certain cancer cells. These biological activities are mediated by type I interferon receptors on the surface of the cancer cells which, when stimulated, initiate various signal transduction pathways leading to reduced proliferation and/or the induction of terminal differentiation or apoptosis. IFNα has been approved by the FDA for the treatment of several cancers including melanoma, renal cell carcinoma, B cell lymphoma, multiple myeloma, chronic myelogenous leukemia (CML) and hairy cell leukemia. A “direct” effect of IFNα on the tumour cells is mediated by the IFNα binding directly to the type I IFN receptor on those cells and stimulating apoptosis, terminal differentiation or reduced proliferation. One “indirect” effect of IFNα on non-cancer cells is to stimulate the immune system, which may produce an additional anti-cancer effect by causing the immune system to reject the tumour.
Unfortunately, the type I interferon receptor is also present on most non-cancerous cells. Activation of this receptor on such cells by IFNα causes the expression of numerous pro-inflammatory cytokines and chemokines, leading to toxicity. Such toxicity prevents the dosing of IFNα to a subject at levels that exert the maximum anti-proliferative and pro-apoptotic activity on the cancer cells.
Ozzello et al. (Breast Cancer Research and Treatment 25:265-76, 1993) described covalently attaching human IFNα to a tumour-targeting antibody, thereby localizing the direct inhibitory activity of IFNα to the tumour as a way of reducing tumour growth rates, and demonstrated that such conjugates have anti-tumour activity in a xenograft model of a human cancer. The mechanism of the observed anti-cancer activity was attributed to a direct effect of IFNα on the cancer cells, since the human IFNα used in the experiments did not interact appreciably with the murine type I IFN receptor, which could have lead to an indirect anti-cancer effect. Because of this lack of binding of the human IFNα to the murine cells, however, the authors could not evaluate the toxicity of the antibody-IFNα conjugate relative to free INFα. These authors used a chemical method to attach the IFNα to the antibody.
Alkan et al., (Journal of Interferon Research, volume 4, number 3, p. 355-63, 1984) demonstrated that attaching human IFNα to an antibody that binds to the Epstein-Barr virus (EBV) membrane antigen (MA) increased its antiproliferative activities towards cells that express the EBV-MA antigen. This increased potency was dependent on both antigen expression by the target cells and the binding specificity of the antibody. The cell line tested was the cancer cell line QIMR-WIL, a myeloblastic leukemia. The authors suggested that the attachment of IFNα to an antibody could be used as a treatment for cancer since it would reduce tumour growth. Alkan et al did not address the potential toxicity of these antibody-IFNα conjugates arising from their interactions with normal, antigen-negative cells.
It is also known that the linkage between an antibody and IFNα may be accomplished by making a fusion protein construct. For example, IDEC (WO01/97844) disclose a direct fusion of human IFNα to the C terminus of the heavy chain of an IgG targeting the tumour antigen CD20. Other groups have disclosed the use of various linkers between the C-terminus of an IgG heavy chain and the IFNα. For example, U.S. Pat. No. 7,456,257 discloses that the C-terminus of an antibody heavy chain constant region may be connected to IFNα via an intervening serine-glycine rich (S/G) linker of the sequence (GGGGS)n, where n may be 1, 2 or 3, and that there are no significant differences in the IFNα activity of the fusion protein construct regardless of linker length.
Morrison et al. (US2011/0104112 A1; and Xuan C, Steward K K, Timmerman J M, Morrison S L. Targeted delivery of interferon-α via fusion to anti-CD20 results in potent antitumor activity against B-cell lymphoma. Blood 2010; 115:2864-71) also disclose IFNα linked to the C-terminus of the heavy chain of a cancer-targeting IgG antibody, with an intervening S/G linker, and observed that the fusion of the IgG and linker to the IFNα reduced the activity of IFNα on cells that did not express the corresponding antigen on the cell surface. The decreased IFN activity of these fusion protein constructs was modest when compared to human non-fusion protein IFNα (free IFNα) acting on human cells, but appeared to be more significant for murine IFNα on murine cells. The decrease in the activity of human IFNα that results from fusing it to the C-terminus of an antibody, as observed by Morrison et al, and in U.S. Pat. No. 7,456,257 is modest and is generally considered to be a disadvantage since it reduces potency of the ligand. This disadvantage was pointed out, for example, by Rossi et al (Blood vol. 114, No. 18, pp 3864-71), who used an alternative strategy of attaching the IFNα to a tumor targeting antibody in such a way that no loss in IFNα activity was observed.
In general the prior art teaches to use a potent IFN and to target this IFN to cancer cells. While this approach results in an increase in activity of the IFN against cancer cells, it does not address the issue of activity of the IFN on normal “off-target” cells. In prior art examples referred to above, the human IFNα portion of the antibody-IFNα fusion protein maintained a high proportion of native IFNα activity when exposed to human cells that do not express the corresponding antigen on their cell surfaces. This activity may lead to toxicity arising from the activation of non-cancerous, normal (“off target”) cells by the IFNα portion of the fusion protein. Accordingly, there exists a need to decrease the “off-target” activity of ligand-based drugs, while retaining the “on-target”, therapeutic effect of such ligands. The maintenance of target-specific ligand activity and at the same time a reduction in non-target toxicity of ligand-based therapeutic agents would create a greater therapeutic concentration window for therapeutically useful ligands. It would for example be desirable to use human IFNα in a form such that its activity can be directed to the cancer cells while minimizing its effects on normal human cells. Ideally the type I interferon receptor on the cancer cells would be maximally stimulated, while the same receptor on non-cancerous cells would experience minimal stimulation. There is a need to target human IFNα to the cancer cells in such a way that it has dramatically more activity on the cancer cells, which display the antigen, than on the normal cells, which do not display the antigen. The same logic applies to other potentially therapeutic ligands, e.g. other cytokines, peptide and polypeptide hormones, chemokines, growth factors, apoptosis-inducing factors and the like.